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Reusser, Lucas Jonathan

Geology

2005

MS

¹⁰Be measured in over 80 samples collected from fluvially eroded rock within Holtwood Gorge quantifies, for the first time, when, how fast, and why the Susquehanna River incised through bedrock along the east coast of North America, one of the most prominent and ancient passive margins. Although the rate at which large rivers incise rock is a fundamental control on the development of landscapes, very little is known about how quickly such incision occurs either in tectonically active environments or along passive margins. Exposure ages of fluvial carved bedrock strath-terraces preserved within Holtwood Gorge demonstrate that even along the Atlantic passive margin, large rivers are capable of incising through rock for short periods of time at rates approaching those recorded in the tectonically active Himalaya. Beginning between ~45 ka and ~35 ka, rates of incision accelerated dramatically, and continued at a rapid rate until ~14 ka. This phase of rapid incision, also measured within Mather Gorge along the Potomac River located -100 km to the south, correlates well with a period of cold and stormy climate recorded by the GISP2 ice core, central Greenland. Unstable climate during the late Pleistocene increased the frequency and magnitude of flood events capable of exceeding thresholds for bedrock erosion, thus enabling both the Susquehanna and Potomac Rivers to incise quickly into rock. Rates of incision during the late Pleistocene, constrained with ¹⁰Be, range from 0.8 to 004 m/ky between ~45 and ~14 ky, and are almost two orders of magnitude faster than long-term estimates of integrated bedrock incision rates (~12 m/My) since the middle Miocene along the Atlantic passive margin. Discordance between short- and long-term rates indicates that incision through bedrock on this passive margin occurs episodically. Driven by long-term flexure of the Atlantic passive margin by offshore deposition of sediment, the lower reaches of the river have remained oversteepened since the Miocene, increasing stream gradients and the potential for incision. The high rates of late Pleistocene incision inferred using data collected for this project represent a pulse of erosion during an ongoing period of river adjustment over geologic time scales. This research provides a framework for future investigation into the style and tempo at which rivers elsewhere incise through bedrock. Results from a variety of spatial tests further our understanding of how and when large passive rivers incise rock and show that the age signal preserved in terrace rocks is consistent and interpretable. Cosmogenic isotopes allow us to measure rates of incision over millennial time scales, an appropriate time frame for considering the influences of changing boundary conditions during glacial-interglacial cycles. Similar studies conducted along additional rivers draining the east coast of North America, as well as passive margins around the globe, will help us to understand better how fluvial incision through these resistant channel reaches modifies ancient terrains.